Update fielderator tests to account for small diff in reals.

These were failing on an Ubuntu 32bit utopic (14.10) system with gcc 4.9.1 due
to a very small (0.00000000000000043) difference between two instances of
17.23.

Update the comparison to find the absolute difference between the two reals and
ensure the difference is less than 1e-8.
Merge pull request #892 from thomasvandoren/robustify-real-comparison

Update fielderator tests to account for small diff in reals.

These were failing on an Ubuntu 32bit utopic (14.10) system with gcc 4.9.1 due
to a very small (0.00000000000000043) difference between two instances of
17.23.

Update the comparison to find the absolute difference between the two reals and
ensure the difference is less than 1e-8.